Phosphate fertilizers



March 30, 1954 v J. H. HUDSON 2,573,795

PHOSPHATE FERTILIZERS Filed Aug. 2o, 1951 In uentor V J o /1/7 /fe/e/- Hudson,

\ B Z Z WX/14AM, l

' Attorneys.

Patented Mar. 30, 1954 UNITED STATES PATENT OFFICE PHOSPHATE FERTILIZERS Claims priority, application Great Britain September 12, 1950 11 Claims.

This invention relates to the production oi phosphatic fertilizers from mineral rock phosphate.

Reference is made to copending application Serial No. 242,625, filed August 20, 1951, by Nathaniel Percy, which deals with subject matter analogous to that of the present invention.

Numerous processes have been suggested for producing a compound fertilizer by treating mineral rock phosphate with nitric acid, in one of which processes the reaction product is treated with ammonia. In all these processes however diiiiculties arise owing to the plastic nature of the resulting mass.

Many proposals have been made with a view to overcoming these diiiiculties. For example, it has been proposed to react mineral rock phosphate with a mixture oi nitric and sulphuric acids or of nitric acid and alkali sulphates in attempts to obtain a reaction product which may be satisfactorily granulated to give a compound fertilizer of desired grading which may be packed in bags and stored without deterioration.

It would appear that these results have not yet been achieved to the extent desirable in a ccmmercial product.

We have found that the desired results may be obtained by the process of the present invention.

According to the present invention there is provided a process for the production of granular compound phosphate fertilizers which comprises the steps of reacting ground mineral rock phosphate with insuiicient aqueous nitric acid for the complete conversion to mono-calcium phosphate of the tri-calcium phosphate content of the mineral rock, the reaction with the acid being carried out in two stages, the rst being of short duration not exceeding 2 minutes under conditions of rapid mixing and during which the whole of the required acid is added, and the second for a period of about 12 to 15 minutes under conditions of moderate agitation; then adding with continued mixing over a period of about minutes, aqueous ammonia to convert substantially all the mono-calcium phosphate produced in the reaction with nitric acid to di-calcium phosphate, while maintaining the mass at a temperature of not more than 70 C.; adding with continued mixing over a period of from 5 to 10 minutes, ground ammonium sulphate in an amount not less than that required to convert unreacted calcium nitrate to calcium sulphate and ammonium nitrate; adding, with continued mixing, dried material from the iinal step of the process in such quantity depending on the concentration of the nitric acid and aqueous ammonia employed to give a mixture having a Water content of between 6% and 12% by weight; said mixture being then passed to a rotary drier or driers in which the granulation is completed and the granules are dried by a counter-current stream oi hot gas so that the temperature of the granulated material leaving the drier or driers is not higher than 100 C.

The granulated material leaving the drier or driers is passed to conventional screens for grading as desired, the oversize being crushed and the iines, with any additional graded material necessary to make up the desired quantity, are returned to the appropriate step of the process. in order to minimise the tendency of the hot granules leaving the drier or driers to agglomerate it is preferred to cool them to about room temperature, for example, by passing them through a rotary cooler, before screening.

The quantity of nitric acid employed will, of course, vary with the tri-calcium phosphate content of the mineral rock, the nature of the impurities present, the degree of conversion desired and on the iineness to which the rock is ground. For example, when using mineral rock containing about by Weight of tri-calcium phosphate, satisfactory results may be obtained by using between 50% to 75% by weight of acid, in terms of HNOa, on the total weight of the mineral rock phosphate being reacted.

The use of a quantity of nitric acid corresponding to the lower limit of the above-mentioned range will give a conversion of about 55% of the tri-calcium phosphate content of the mineral rock and a quantity of acid corresponding to the upper limit will give about 96% conversion. It has been found undesirable to use a quantity of acid which will give a conversion of more than 96% as above this the reaction product becomes much more fluid and in consequence diiiiculties arise in the operation of the process, particularly as it becomes necessary to increase the amount of dried material which is returned to the process from the nal step and/or to prolong the drying. In addition it has been found that if the amount of acid is increased above that required to give 96% conversion of the tri-calcium phosphate content of the mineral rock there is substantially no further increase in the proportion of the total P205 in the iinal product which is soluble in 2% citric acid.

In order to obtain efficient utilisation of the nitric acid it is desirable for the mineral rock 55 to be ground to pass A. S. T. M. Standard mesh sieve No. 50 (297e) with at least 75% by weight to pass A. S. T. M. Standard mesh sieve No. 120 (12S/i) Preferably, the rock is ground so that at least passes A. S. T. M. Standard mesh sieve No. 120 (125e) and at least 50% passes A. S. T. M. Standard mesh sieve No. 200 (74p). It has been found that with the mineral rock ground to less than 297s and at least 75% less than 125,1 the quantity of nitric acid required to give a desired degree of conversion of the tri-calcium phosphate content may be estimated from the analysis of the roel; with an accuracy within normal limits of error by allowing the full theoretical quantities of acid required to react with the impurities which are calcium or magnesium compounds other than calcium fluoride and 70% only of the theoretical quantities required to react with other impurities, such as aluminium, iron and sodium compounds, with the exception of silica and organic matter.

For example with Florida rock of the analysis,

74.8 parts of Ca3 PO02 require x74.8X0.06=58.3 parts HNO;

2 6.45 parts of CaC03 require gigxdi 8.1 parts HNO3 2\ 0.87 parts oi CaSO4 require -TBXQM 0.8 parts ENOa 2 0.22 parts of MgO require -A %3X0.22

0.7 parts HNOs ext3 1.03 parts of A1203 require 0.93 parts oi l`e203 require -(KXOHXOJ 1.5 parts HNOg 0.64 parts of NazO require GE-XOMXOJ 0.9 parts HNOa 73.0 parts HNOs The concentration of the nitric acid may be varied but it has been found convenient to use acid having a concentration between and 65% cy weight of aqueous solution. Preferably the ccncentration should be `oetween and 55% by weight in order to keep the water content low and to avoid the losses of acid as fumes when higher concentrations are used.

The quantity of aqueous ammonia used will, of course, depend upon its concentration and the degree of conversion to mono-calcium phosphate of the tri-calcium phosphate content of the mineral rock.

The concentration of the aqueous ammonia may be Varied over a Wide range, for xample from 20% to 40% by weight of ammonia, but it is preferred to employ a concentration of about 30% by weight.

The temperature of the reaction mass during the addition of aqueous ammonia should not exceed 70 C. and should preferably be in the range of 50 to 60 C.

The ammonium sulphate used in the process of the present invention may be any commercial crystalline ammonium sulphate. The ammonium sulphate should be ground preferably to less than 0.5 mm.

The quantity of dried material from the iinal step or" the process which is returned to the step of the process before the drier or driers, and which will normally include the nes from the final screening, will, as before mentioned, be such as to give a product having a moisture content of between 6% and 12% by weight. It has been found convenient to arrange for the concentration of the nitric acid and aqueous ammonia used in the process be such that the quantity of dried material returned is between 1 and 4 times by weight of the total of the other materials fed to the process.

The process of the present invention may be carried out batchwise, but it is preferred to opcrate in a continuous manner.

Batchwise operation may be carried out in a Vessel which is provided with suitable means, for example, a stirrer or other agitator, or agitators, by which the first stage of reacting the mineral rock with the nitric acid may be carried out under the desired conditions of rapid mixing and the subsequent operations of moderate agitation and continued mixing may be effected by means of a change speed gear or by the operation of a second stirrer and/or agitators.

For operating the process in a continuous manner it is preferred to arrange a separate mixer, which is preferably a paddle mixer, for the first stage of the reaction of the mineral rock with the nitric acid followed by a mixer, which is preferably a continuous or interrupted screw mixer, in which the second stage of the reaction of the mineral rock with the nitric acid is carried out and further mixers, which are preferably paddle mixers, in which the continued mixing with the addition of aqueous ammonia, ammonium sulphate and returned dried material, is carried out. The mixer in which the aqueous ammonia is added is provided with means, for example a water jacket, for maintaining the reaction mass at a temperature not exceeding C.

If desired, some adjustment of the temperature of the reaction mass may be arranged to take place in the screw mixer in which the second stage of the reaction of the mineral rock phosphate with the niti'ic acid is carried out, for example by providing a water jacket to at least a portion of the length of the screw mixer at the end from which the reaction mass passes to the paddle mixer in which the addition of aqueous ammonia takes place.

1n a preferred continuous manner of carrying out the process of the present invention it has been found convenient to arrange the continuous or interrupted screw mixer, in which the second stage of the reaction of the mineral rock with the nitric acid is conducted, and the paddle mixer, in which the addition of aqueous ammonia is made, to be in a common casing and to have a common shaft.

Similarly it has been found convenient to arrange the paddle mixers, in which the ammonium sulphate and returned dried material are respectively added, to be in a common casing and to have a common shaft.

lt will be understood that in all cases the various mixers will be proportioned and their speeds arranged to give the desired reaction and mixing times for each stage and/or step of the process and to give the desired rate of now of the materials through the plant.

5, A preferred form of apparatus for carrying out the process of the presentV invention is shown diagrammatically in the accompanying drawing.

Referring to the drawing numeral i is a hopper from which ground mineral rock phosphate is fed to the paddle mixer 2 to which aqueous nitric acid is also ied through pipe 3 at the desired rate. The longitudinal sha-ft of the paddle mixer Z is driven, by any convenient means, at high speed in order to achieve a short time, rapid mixing of the mineral rock and nitric acid. The reaction mixture passes from the paddle mixer 2 into the screw mixer il, the shaft or" which is driven at moderate speed to give a moderate agitation to the reaction mixture. The reaction mixture passes from the screw mixer l into the paddle mixer 5 into which there is also fed aqueous ammonia through the pipe 5. The screw mixer i and the paddle mixer 5 are indicated as being in a common casing and having a common shaft. The paddle mixer 5 is provided with means, not shown, for maintaining the reaction mass at a temperature not exceeding 70 C. The material leaving the paddle mixer 5 passes into the paddle mixer l which essentially comprises two sections into the first of which ground ammonium sulphate is fed at il at the desired rate and into the second section is fed at 5 the desired quantity of dried material from the nal step of the process. rThe material leaving the paddle mixer 'i passes into the drier i5 in which granulation is completed and the granules are dried by hot gas which enters the drier at i l and leaves it at i2. The granulated material leaves the drier at l5 and passes to a cooler and/or conventional screens not shown on the drawing.

The continuous manner of carrying out the process of the lpresent invention is described in the following examples by reference to the accompanying drawing.

Example 1 Mineral rock phosphate ground to pass A. S. T. M. Standard mesh sieve No. 50 (297/0 and '75% to pass A. S. T. M. Standard mesh sieve No. 120 (125s) was fed at the rate of 100 pounds per hour from a hopper l into the paddle mixer il. Nitric acid in a concentration of 50% by Weight of aqueous solution was also fed into the paddle mixer 2 at the rate ci 120 pounds per hour by means of the pipe 3. The paddle mixer shaft was driven at high speed and the retention time ci the reactants in the mixer was one minute. The fluid reaction mass leaving the paddle mixer 2 passed into the screw mixer il the shaft of which was driven at moderate speed to give a retention time of l5 minutes. The reaction mass from the screw mixer i passed into the paddle mixer 5 which was arranged in a common casing and on a common shaft with the screw mixer l, the proportions of the paddle mixer 5 being such as to give a retention time of about minutes. The casing of the combinedmixers i and 5 was provided with a water jacket by means of which the temperature of the reaction mass passing through was reduced to and maintained between 45 to 50 C., particularly during the addition of aqueous ammonia of 30% concentration at the rate of 10 litres per hour which was passed into the paddle mixer 5 by means ci the pipe t. The fluid ammoniated product leaving the paddle mixer 5 was fed into the paddle mixer 'l into the irst section of which there was added at the rate of 55 pounds per hour by means of the pipe 8, ammonium sulphate which had been ground to pass A. S. T. M, Standard mesh sieve No. 35 (0.5 mm). Into the second section of the paddle mixer 'l' there was added, at the rate of 600 pounds per hour by means of the inlet 5, dried material, including fines, from the screening of the dried product. The paddle mixer 'l was so proportioned that the retention time of the materials in each section was of the order of 5 minutes.

The partially granulated product from the paddle mixer 'l had a moisture content of 8.5% and Was passed into the rotary drier iii in which granulation was completed and the granules dried by a stream of hot air which entered the drier at i i and lett it at i2. The granules passed from the drier at i3 at a temperature of 75 C'. and after being passed through an air cooler were graded between i mm. and fi mm. to give 30% by Weight or" granules o1 this grading and about equal quantities of oversize and nes. The oversize granules after crushing were returned with the rines to the paddle mixer i.

On analysis the granulated product gave the following:

Per cent Total P205 15.7 2% citric acid soluble P205 l3.0 Water soluble P205 1.0 Nitrogen 13.0 liree Water 1.5

Example 2 Morocco rock phosphate having the analysis:

Per cent Cac(PO4)2 75.1 CaCOz 8.3 CaFg 3.2 CaSOi 2.6 Wg()l 0.7 Naz() 0.9 F6203 0.3 A1203 6.6 SiOz 0.9 Organic matter 0.8

was ground so that all passed A. S. T. Ivi. Standard mesh sieve No. 55 (297m with 85% to pass A. S. T. M. Standard mesh sieve No. 120 (125s) and 50% to pass A. S. T. M. Standard mesh sieve No. 200 (74M).

The quantity of nitric acid (as 100% HNOa) required for conversion oi the tricalcium phosphate content was estimated, as hereinbefore described, to be 7'? parts by weight of acid per parts by weight oi the rock.

The ground rock was fed at the rate ci" 100 pounds per hour from the hopper i into the paddle mixer 2. Nitric acid of a concentration of 50% by weight of aqueous solution was also fed into the paddle mixer 2 at the rate of 154 pounds per hour by means of the pipe The speed of the mixer shaft was arranged so as to give a retention time of the reactants in the mixer of one minute. The fluid reaction mass passed from the paddle mixer 2 into the screw mixer il, the shaft oi which was driven at moderate speed to give a retention time of l5 minutes. The reaction mass passed from the screw mixer 1 into the paddle mixer 5 which was arranged in a common casing and on a common shaft with the screw mixer d, the proportions of the paddle mixer being such as to give retention time of about 10 minutes. The casing of the combined mixers 4 and 5 was provided with a water jacket by means of which the temperature 0f the reaction mass passing through was reduced to and maintained between 50 C. and 60 C., particularly during the addition of aqueous ammonia of 30% concentration at the rate of 27.5 pounds per hour which was passed into the paddle mixer through the pipe Ei. The fluid ammoniated product passed from the paddle mixer 5 to the paddle mixer 7 into the rst section of which ammonium sulphate, ground to pass A. S. T. M. Standard mesh sieve No. 5 (0.5 mm.) was added by means of the pipe 3 at the rate of 32 pounds per hour. Into the second section of the paddle mixer there was added, at the rate of 800 pounds per hour through the inlet 9, dried material, including fines from the screening of the dried product. The retention time of the materials in each of the two sections of the paddle mixer was about 5 minutes.

The partially granulated product from the paddle mixer had a mixture content of 9% and was passed into the rotary drier lll in which granulation was completed and the granules were dried by a stream of hot air which entered the drier at i i and left it at 2. The granules passed from the drier at I3 at a temperature of about 75 C. and after being passe. through an air cooler were graded between 1 mm. and 4 mm., and gave 25% by weight of granules of this grading, 30% oversize and 45% rines. The oversize granules were crushed and returned with the nes to the paddle mixer '1.

The analysis of the granulated product was:

Per cent Total P205 14.8 2% citric acid soluble P205 13.6 Water soluble P205 1.5 Nitrogen 13.6 Free water 1.5

I claim:

1. A process for the production of granular compound phosphate fertilizers which comprises the steps of reacting ground mineral roch pho.,- phate with insuilicient aqueous nitric acid for the complete conversion to mono-calcium phosphate of the tri-calcium phosphate content of the 't mineral rook, the reaction with the acid being carried out in two stages, the first being of short duration not exceeding 2 minutes Linder conditions of mixing and during which the whole of the required acid is added, and the second for i a period of less than minutes under conditions of moderate agitation; then adding with continued mixing over a period o less than 10 minutes, aqueous ammonia to convert substantially all the mono-calcium phosphate produce in the reaction with nitric acid to cli-calcium phosphate, while maintaining the mass at a temperature not more than 70 C.; adding with continued mixing over a period of less than 10 minutes, ground ammonium sulphate in an amount not less than that required to convert unreacted calcium nitrate to calcium sulphate and ammonium nitrate; adding, with continued mixing, dried material from the final step of the process in such quantity depending on the concentration of the nitric acid and aqueous ammonia employed to give a mixture having a 8 Water content of between 6% and 12% by weight; said mixture being then passed to at least one rotary drier in which the granulation is completed and the granules are dried by a counter-current stream of hot gas so that the temperature of the granulated material leaving the drier is not higher than 100 C.

2. A process as claimed in claim l in which the mineral rock phosphate is ground to pass A. S. T. M. Standard mesh sieve No. with at least by weight of the ground mineral roch ground to pass A. S. T. M. Standard mesh sieve No. 120.

3. A process as claimed in claim 2 in which at least by weight of the ground mineral rock is ground to pass A. S. T. M. Standard mesh sieve No. and at least 50% is ground to pass A. S. T. M. Standard mesh sieve No. 200.

4. A process as claimed in claim l in which the granulated material leaving the drier is screened to desired grading, the oversize granules are crushed and together with the fines, and if necessary with any additional graded material to make up the desired quantity, are returned to the step of the process immediately before the drying step.

5. A process as claimed in claim 1 in which the hot granules leaving the drying step are cooled to about room temperature before screening.

6. A process as claimed in claim 1 in which the quantity of nitric acid used is sufficient to react with the impurities present in the mineral rock and to convert not more than 96% of the tricalcium phosphate content of the rock to monooalcium phosphate.

7. A process as claimed in claim 1 in which the concentration of the nitric acid is between 45% and 65% by weight of aqueous solution.

8. A process as claimed in claim 1 in which the concentration of the aqueous ammonia is between 20% and 40%.

9. A process as claimed in claim l in which the temperature of the reaction mass during the addition of aqueous ammonia is maintained at a temperature in the range of 50 C. to 60 C.

10. A process as claimed in claim 1 in which the ammonium sulphate used is ground to less than 0.5 mm.

1l. A process as claimed in claim 1 in which the quantity of dried material returned from the final step of the process to the step before the drying step is between 1 and 4 times by weight of the total of the other materials fed to the process.

JOHN HERBERT HUDSON.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,854,291 Burdick Apr. 19, 1932 FOREIGN PATENTS Number Country Date 282,330 Great Britain May 3l, 1928 355,637 Great Britain Aug. 27, 1931 391,495 Great Britain May l, 1933 615,476 Great Britain Jan. 6, 1949 

1. A PROCESS FOR THE PRODUCTION OF GRANULAR COMPOUND PHOSPHATE FERTILIZERS WHICH COMPRISES THE STEPS OF REACTING GROUND MINERAL ROCK PHOSPHATE WITH INSUFFICIENT AQUEOUS NITRIC ACID FOR THE COMPLETE CONVERSION TO MONO-CALCIUM PHOSPHATE OF THE TRI-CALCIUM PHOSPHATE CONTENT OF THE MINERAL ROCK, THE REACTION WITH THE ACID BEING CARRIED OUT IN TWO STAGES, THE FIRST BEING OF SHORT DURATION NOT EXCEEDING 2 MINUTES UNDER CONDITIONS OF RAPID MIXING AND DURING WHICH THE WHOLE OF THE REQUIRED ACID IS ADDED, AND THE SECOND FOR A PERIOD OF LESS THAN 15 MINUTES UNDER CONDITIONS OF MODERATE AGITATION; THEN ADDING WITH CONTINUED MIXING OVER A PERIOD OF LESS THAN 10 MINUTES, AQUEOUS AMMONIA TO CONVERT SUBSTANTIALLY ALL THE MONO-CALCIUM PHOSPHATE PRODUCED IN THE REACTION WITH NITRIC ACID TO DI-CALCIUM PHOSPHATE, WHILE MAINTAINING THE MASS AT A TEMPERATURE OF NOT MORE THAN 70* C., ADDING WITH CONTINUED MIXING OVER A PERIOD OF LESS THAN 10 MINUTES, GROUND AMMONIUM SULPHATE IN AN AMOUNT NOT LESS THAN THAT REQUIRED TO CONVERT UNREACTED CALCIUM NITRATE TO CALCIUM SULPHATE AND AMMONIUM NITRATE TO CALCIUM SULPHATE MIXING, DRIED MATERIAL FROM THE FINAL STEP OF THE PROCESS IN SUCH QUANTITY DEPENDING ON THE CONCENTRATION OF THE NITRIC ACID AND AQUEOUS AMMONIA EMPLOYED TO GIVE A MIXTURE HAVING A WATER CONTENT OF BETWEEN 6% AND 12% BY WEIGHT; SAID MIXTURE BEING THEN PASSED TO AT LEAST ONE ROTARY DRIER IN WHICH THE GRANULATION IS COMPLETED AND THE GRANULES ARE DRIED BY A COUNTER-CURRENT STREAM OF HOT GAS SO THAT THE TEMPERATURE OF THE GRANULATED MATERIAL LEAVING THE DRIER IS NOT HIGHER THAN 100* C. 